This application claims the benefit of Korean Application No. 00-20069, filed Apr. 17, 2000, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an objective lens and an optical pickup device using the same, and more particularly, to an objective lens which effectively compensates for the chromatism generated at an objective lens due to a change in the output of a light source when a mode is shifted, and to an optical pickup device using the same.
2. Description of the Related Art
As the storage capacity of optical storage devices increases, the quality of reproduced signals becomes more affected by reproduction conditions. In the case of an optical pickup using a blue laser light source having a short wavelength, the reproduced signal deteriorates due to chromatism generated from an objective lens.
In general, for the glass and plastic materials used to make an objective lens of an optical pickup device, as a wavelength of a light source decreases, the refractive index thereof increases. Also, the rate of change in the refractive index with respect to the change in wavelength increases as the wavelengths decrease so that the amount of chromatism generated corresponding to the amount of change in the wavelength increases. The objective lens used for an optical pickup device of an optical storage device is usually of an aspherical mono lens type formed of plastic or glass. In an optical pickup device using a blue laser light source of 400 nm, the deterioration in quality of a reproduced signal caused by chromatism due to a change in the wavelength of a laser light source is unavoidable.
FIG. 1 is a graph showing the change in the refractive index due to a change in the wavelength for each type of lens material. Referring to the drawing, it can be seen that, as the wavelength becomes shorter, the refractive index for each material changes rapidly. The change in the refractive index due to a change in the wavelength greatly affects the performance of an objective lens used for an optical storage device. Chromatism due to a change in the oscillation wavelength of laser light changes the working distance between an objective lens and a disk, which means that the chromatism due to the change in wavelength can be compensated for.
In the case of an optical storage device capable of reproduction and recording, the power of the laser light used for reproduction and recording varies. The change in power causes changes in wavelength of about xc2x12 nm with respect to a reference wavelength of laser light. Accordingly, chromatism due to the change in the wavelength is generated at an objective lens designed for the reference wavelength. Such chromatism can be compensated for by adjusting the working distance between the objective lens and the disk. Adjustment of the working distance is performed in a process in which a reproduced signal is made optimal by a focus servo mechanism. However, after the chromatism is generated, since a duration on the order of about tens of microseconds is required for operating a circuit of the focus servo mechanism, the quality of a signal reproduced or recorded for the above time deteriorates.
Thus, in a reproducing and recording optical storage device, an objective lens is needed which can reduce or compensate for the generation of chromatism when the wavelength of laser light changes in a range of at least xc2x12 nm due to a shift in mode.
FIG. 2 shows the structure of a single objective lens 2 used in a conventional optical pickup device for an optical storage device. FIG. 3 shows the property of chromatism for selected wavelengths of an objective lens in a state in which the working distance between the objective lens and the disk is fixed. Here, it is assumed that the objective lens is for a DVD (digital versatile disc) and has a structure having a reference wavelength of 650 nm and an NA (numerical aperture) of 0.6. As can be seen from FIG. 3, even when the wavelength changes by xc2x12 nm with respect to the reference wavelength of 650 nm, since the RMS (root means square) chromatism is less than or equal to 0.04 xcex without adjustment of focus, no special problem is created in the performance of the optical pickup device.
Table 1 shows data of each of surfaces of the disk 1 and objective lens 2. Table 2 shows the amount of defocus and the wave front chromatism for each wavelength.
The amount of defocus in Table 2 is the amount of distance changed adjusted from (3.32 mm) in Table 1 so that the distance between the objective lens 2 and the minimum wave front chromatism.
FIG. 4 shows a property of chromatism for each wavelength with respect to an objective lens 2 designed under the conditions of the wavelength being 405 nm and the NA being 0.6. As shown in FIG. 4, in a reference wavelength of 405 nm, a mere change of about xc2x12 nm increases the RMS wave front chromatism over 0.1xcex. As a result, such a wave front chromatism shows that the objective lens 2 cannot be applied to an optical pickup of an optical storage device. Data of each surface according to the above optical design is shown in Table 3 below. Table 4 shows the amount of defocus and wave front chromatism for each wavelength.
As a widely used method for compensating for the chromatism due to the change in wavelength as above, a conventional objective lens 2 is made by assembling two aspherical lenses formed of different materials. However, it is difficult to manufacture an objective lens 2 by injection molding each of the two aspherical lenses and combining the two units. In particular, when chromatism is to be corrected by using two aspherical lenses, a low-distribution lens having a positive power and a high-distribution lens having a negative power are manufactured respectively using flint-based glass and crown-based glass as materials. Then, the respective lenses are combined or coupled with a particular air gap. However, in the conventional objective lens 2, since the respective aspherical lenses must have a distribution of over 30, the kinds of lens material to be injection molded are quite limited so that it is difficult to choose a material for each of the lenses. The term distribution refers to the variation of the refractive index of an optical material with respect to changes in wavelength.
To solve the above problems, it is an object of the present invention to provide an optical pickup device which can be easily manufactured and assembled and in which chromatism due to a change in wavelength can be corrected.
It is another object of the present invention to provide an optical pickup device in which the material of a lens for correcting chromatism due to a change in wavelength can be easily chosen.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
Accordingly, there is provided an objective lens comprising an aspherical lens facing a disk and having a predetermined first focal distance and a predetermined numerical aperture, and a holographic lens installed adjacent to the aspherical lens having a second focal distance longer than that of the first focal distance and having a diffraction lattice to compensate for the chromatism due to a change in wavelength of laser light having a wavelength of 670 nm or less.
Also, there is provided an optical pickup device comprising a light source to generate a laser light to a disk, an objective lens including an aspherical lens facing a disk and having a predetermined first focal distance and a predetermined numerical aperture, and a holographic lens installed adjacent to the aspherical lens having a second focal distance longer than that of the first focal distance and having a diffraction lattice to compensate for the chromatism due to a change in wavelength of laser light having a wavelength of 670 nm or less, a beam splitter provided between the light source and the objective lens, and a photodetector to receive light reflected by the disk and having passed through the beam splitter.
Further, there is provided an optical pickup device comprising first and second light sources to generate laser lights having different wavelengths to a respective one of two different kinds of disks, an objective lens including an aspherical lens facing a disk and having a predetermined first focal distance and a predetermined numerical aperture, and a holographic lens installed adjacent to the aspherical lens and having a second focal distance longer than that of the first focal distance and having a diffraction lattice to compensate for the chromatism due to a change in wavelength of laser light having a wavelength of 670 nm or less, a first beam splitter provided between the first light source and the objective lens to pass the laser light emitted from the first and second light sources to the disk, a second beam splitter provided between the first beam splitter and the first light source to pass the laser light emitted from the second light source to the disk, and a photodetector to receive light reflected by the disk and passing through the first beam splitter.
There is further provided an optical pickup device in which the disk is one of a disk type including a DVD and an HD-DVD.
There is still further provided an optical pickup device in which the aspherical lens has a positive power and the holographic lens has either a positive or a negative power.
Also, it is preferred in the present invention that the aspherical lens has positive power and the holographic lens is positive or negative power.